Stepper motors are currently being used in automotive instrument clusters to display items such as vehicle speed, engine RPM, fuel level, and engine temperature, for example. Stepper motors utilized in an open loop system require initialization strategies to find a reference, or zero, position. If the zero reference position is not found accurately at key events such as power on reset, running reset, or ignition on/off sequencing, then misleading or inaccurate information can be conveyed to the driver. Detection of the zero position enables accurate position calibration. Typically, the detection of the zero position involves the detection of a stepper motor condition such as a stall condition, for example.
Original strategies rotated the stepper motors towards a fixed zero position reference stop at a fixed velocity for a fixed duration of time to ensure that the pointer attached to the stepper motor started at a known repeatable position. This strategy results in an excessive amount of time required for the reset homing to occur and can also result in the pointer bouncing repeatedly on the stop.
Recent strategies have incorporated back electro-motive-force (EMF) detection to signal when the pointer has stalled against the stop. This involves repeatedly driving one coil of the stepper motor and monitoring the other coil for the induced voltage.
One current solution is to use the built-in hardware of a particular microcontroller that integrates and accumulates back EMF. This strategy uses a fixed time period and a threshold comparison to detect if the pointer has reached the stop. Manufacturing processes for this solution prevent the gain from the detection circuitry to be within acceptable ranges to provide a viable and robust detection algorithm.
It would be desirable to have a detector device and a method for detecting a stall condition in a stepper motor, wherein the characteristics of the back EMF are identified directly from the waveform and not derived and therefore subject to characteristics of the underlying hardware.